This invention relates to acoustic transducers for generation, detection, etc. of ultrasonic waves, in particular to ultrasonic transducers utilizing a zinc oxide (ZnO) thin film as a piezo-electric material.
Zinc oxide (ZnO) is widely utilized in high frequency ultrasonic transducers, because its electromechanical coupling coefficient of the longitudinal oscillation in the direction of the thickness is for bulk as large as 0.30 and thin films thereof can be fabricated relatively easily. They are fabricated by chemical vapor deposition (CVD), sputtering methods, etc. The sputtering methods are predominantly utilized. Among the sputtering methods various thin film formation methods are known, such as direct current sputtering, high frequency sputtering, sputtering with zinc oxide target, reactive sputtering by which sputtering is effected in an oxygen atmosphere by using a zinc plate as a target. However, by any method, deterioration of characteristics due to formation in thin film is inevitable.
By these methods, polycrystalline zinc oxide films are formed on amorphous substrates with c-axis orientation. In this case, zinc oxide films, which are better in crystallinity and c-axis orientation with respect to the substrate, have larger electro-mechanical coupling factors. Consequently it is important to form zinc oxide thin films, with high crystallinity and c-axis orientation. A number of studies are performed in order to obtain optimum formation conditions with this respect.
On the other hand, for constructing a piezo-electric element, a pair of electrodes for applying a voltage thereon are necessary and gold Au, which is excellent in conductivity and stability, is most widely utilized as electrode material therefor. However, since adhesion strength of a gold film deposited directly on a substrate of quartz glass, etc. is small, a layer of chromium Cr, titanium Ti, aluminium Al, etc. is needed to make the gold film stick to the substrate.
It has been already discussed in an article entitled "ZnO Acoustic Transducers Utilizing Crystalline Gold Substrate" by R. Wagers, G. Kino, et al., Proc. IEEE Ultrasonics Symposium 194 (1972) that the sensitivity of zinc oxide thin film ultrasonic transducers utilizing gold as electrode material depends on properties of the gold film. However, formation conditions for the gold film, which is required for constructing an ultrasonic transducer having a high electro-mechanical coupling factor, have not yet quantitatively been well known.